The invention relates to an EUV lithography device comprising: an illumination device for illuminating a mask at an illumination position in the EUV lithography device and a projection device for imaging a structure provided on the mask onto a light-sensitive substrate. The invention also relates to a method for processing an optical element.
In EUV lithography devices/projection exposure systems, reflective elements, in particular mirrors, are typically used as optical elements because no optical materials having a sufficiently large transmittance are known at the wavelengths which are used during exposure operation and which are typically between 5 nm and 20 nm. It is necessary to operate the mirrors in a vacuum with such projection exposure systems because the service-life of the multiple-layer mirrors is limited by contaminating particles or gases. The masks used in EUV lithography devices (also known as reticles) have a structure similar to the multiple-layer mirrors, that is to say, they also have a multiple-layer coating which is configured to reflect radiation in the EUV wavelength range.
Such a multiple-layer coating generally comprises alternating layers of materials having a high and low refractive index, for example, alternating layers of molybdenum and silicon, whose layer thicknesses correspond to each other in such a manner that the coating fulfils its optical function and a high level of reflectivity is ensured. There is typically applied to the multiple-layer system a cap layer which protects the layers below, for example, from oxidation and which may comprise, for example, ruthenium, rhodium, palladium, etc. Masks for EUV lithography may be in the form of shadowing masks, that is to say, they have at the upper side of the cap layer structures which do not reflect the exposure radiation. Those structures may comprise, for example, chromium or other metals. Alternatively, the masks may also be in the form of phase masks. In that case, the uppermost layers of the multiple-layer coating and/or the cap layer may have a suitably adapted layer thickness or there may be provided on the phase mask additional layers, for example, comprising silicon, molybdenum or ruthenium.
It is known that all types of atoms, molecules and compounds, in particular hydrocarbons, which are present in the EUV lithography device have a given probability of reaching the surfaces of the reflective optical elements (including the mask) and becoming bonded at that location. EUV radiation present during exposure operation and the photoelectrons produced thereby, in particular secondary electrons, typically involve the risk that those atoms, molecules or compounds may react with the optical surfaces and become deposited thereon, which results in increasing damage and associated loss of reflection of the mirrors. In the mask, there additionally occurs the problem that the structures to be imaged are covered by the impurities and the shape thereof is changed or, in the case of phase masks, the phase position is changed. If attempts are made to remove those impurities by cleaning the entire surface of the mask, undesirable defects in chromium structures or phase-shifting structures may further also be produced. When structures of a contaminated mask, or a mask damaged in some other manner, are imaged, those defects are transmitted directly to the image of the structures on the light-sensitive substrate (layer of photo-sensitive resist on the wafer). In an extreme case, the entire exposed wafer can thereby become unusable and must be rejected.
US 2007/0132989 A1 by the same Applicant discloses the provision in a projection exposure system of a test system which is configured to image a sub-region of the surface or the entire optical surface of an optical element of the system to an enlarged scale, the optical elements also being able to be arranged in a vacuum. The test system can further also have a device for removing the contamination from the tested optical surface, in particular by a cleaning gas being supplied to the surface, plasma cleaning, etc.
U.S. Pat. No. 6,829,035 B2 discloses a device for producing semiconductor components, which has a lithography station for imaging a pattern onto a semiconductor wafer. A mask cleaning station is further provided in order to receive the mask from the lithography station, to clean contamination from the mask and subsequently to direct the cleaned mask back to the lithography station. In this instance, the mask can be moved back and forth using a robot between the lithography station and the mask cleaning station. The cleaning station may comprise a laser in order to process the mask in a locally resolved manner. It is further possible to direct process gases mixed with (water) vapour onto the surface. The vapour condensed on the surface to form a liquid is evaporated in an explosive manner by the laser radiation striking the surface, whereby the contaminating particles are detached from the mask. The detached particles are drawn off the surface by using one or more gas outlet ducts in order to prevent them from being able to become fixed to the surface again.
PCT/EP2007/009593 by the same Applicant describes a method for removing a contamination layer from the surface of optical elements which are arranged in an EUV lithography system for microlithography. During the method, a stream of a cleaning gas which particularly contains atomic hydrogen is directed onto the surface. At the same time, the thickness of the contamination layer is checked and the gas stream is moved relative to the surface in accordance with the measured thickness. In that manner, cleaning with cleaning gases such as atomic hydrogen is intended to be possible without the surface of the optical element being damaged by the cleaning gas.
Whereas cleaning of optical elements can be carried out in situ, that is to say, without being disassembled from the projection exposure system, the mask and the wafer are typically removed from the lithography system for inspection and/or for repair and supplied to a separate inspection and/or processing device provided therefor. This device may be constructed, for example, as in U.S. Pat. No. 6,855,938 which describes an electron microscopy system which is configured simultaneously to carry out inspection or imaging of the object and processing in that there is also provided an ion beam processing system in addition to the electron microscopy system.
US 2003/0051739 A1 discloses a generator for providing and activating gases for cleaning carbon contamination from the surfaces of EUV-reflective optical elements. The gases provided are excited or activated by low-energy electrons and are subsequently directed to the surfaces to be cleaned.